The present invention relates to a device for determining vibration characteristics of vibrated, supported, generally round, substantially ellipsoid articles, such as eggs, comprising:
a hammer with a handle and head, for tapping and thereby acoustically vibrating such article,
a handle driving element for reciprocating the hammer generally in a plane around an axis in the handle,
a microphone arranged immediately adjacent to and directed to the article, for picking up acoustic vibrations generated by the article, and
a signal processing means for processing the signals picked up by the microphone for determining vibration characteristics of the article.
Such devices have long been known. In U.S. Pat. No. 3,067,605 a tapping device for determining cracks and fractures in the egg shell are described, with which in particular the extent of rebound after tapping is determined. In NL9401388, by tapping an egg, for instance by means of a ball falling onto it, the egg""s reaction thereto, in particular the acoustic resonance, is measured with a microphone arranged nearby. In xe2x80x9cAssessment of some physical quality parameters of eggs based on vibration analysisxe2x80x9d, P. Coucke, Catholic University Leuven, March 1998, an extensive analysis is shown both of publications from the past and of measurements on resonance modes, with which well-defined features and characteristics of eggs can be traced. In JP10227766 a tapping device for eggs is described, wherein the tapping rod consists of two parts connected by a spiral spring, whilst the reciprocating movement involves an interplay between pushing around by a stop and a rebounding of a supporting spring.
Also for determining the properties of fruits, in particular determining the firmness of apples from the acoustic resonance thereof, for instance described in xe2x80x9cNon-destructive firmness measurements of applesxe2x80x9d, Armstrong et al., Am. Soc. Agr. Eng., June 1992, the principle of tapping with a hammer and picking up the sound signals with a microphone is applied.
All of these examples show that solutions are being sought to the problem of effecting resonances in a manner as uniform as possible, and subsequently measuring thus generated signals in a manner as accurate as possible. An important problem is formed by the tapping itself. It has been demonstrated that in many cases tapping is a repetitive phenomenon, to be qualified as bouncing. This is used, for instance, in EP738888, also in applicant""s name. In this document, essentially the reaction of impact on the tapping element itself is examined. If any resonances can be measured there at all, these will often be disturbed by the repetitive tap mentioned.
The present invention has resulted in a device which contemplates a further improvement and is characterized in that the handle adjacent the axis consists of an arm portion to be driven which is connected, through a hinge element, with a handle end having at the extremity thereof a mount having therein a ball as a head, while at least the hinge element and the handle end form a hammer rod in one piece.
It has been found that, with this, a single tapping pulse is obtained. Such a pulse is very suitable for further signal processing, in particular for determining the frequency spectrum associated with such a pulse. Such a spectrum contains all possible information about the resonance modes which, as a result of tapping, are generated by the tapped articles.
In further embodiments of the present invention, the device is characterized in that the plane passes through the long axis of the article, that at least a single microphone is arranged in said plane, or through the long axis in a second plane, substantially perpendicular to said plane.
Another embodiment of the device according to the invention is characterized in that the hammer rod and the arm portion constitute one whole, with the hammer rod forming a leaf spring portion having a spring constant k between 1.2 and 1.6 N/m. The pulses herewith obtained are very suitable and short, and thus result in a corresponding, highly suitable frequency spectrum.
In a particular embodiment of the invention, the device is characterized in that the handle driving element further comprises a holder with pin hole for a pin perpendicular to the first plane and through the arm portion, with an electromagnet attached to the holder for reciprocating the hammer generally in said plane, with a magnet included in the arm portion adjacent the electromagnet, and with a stop element for the arm portion during the forward movement.
In a further particular embodiment, the device is characterized in that the handle driving element further comprises a stop for interrupting the return movement of the hammer, and in a still further embodiment that the ball is made of steel, and that the handle driving element further comprises a holding element with which the hammer is held after a return movement, the holding element consisting of a stop block for the leaf spring portion and a holding magnet for the ball.
In a still further embodiment, the device is characterized in that the hammer rod is further coupled with the arm portion by means of a bistable switch, the switch having a first and a second snap position, while the hammer rod is movable either into the first snap position or into the second snap position. In this way, it is suitably ensured that the hammer, after tapping, is immediately retracted and remains retracted, and bouncing is thereby avoided.
In a particular embodiment, the device is characterized in that the hammer rod in the forward movement is switched to the first snap position, and in the return movement is switched to the second snap position.
With such a device, the manner of tapping can be set very accurately. Especially in sorting large numbers of articles varying slightly in shape, this is highly advantageous, and very reliable results are obtained.
Further, the invention comprises a method for determining vibration characteristics of vibrated articles such as eggs, characterized in that the tapping of the articles is performed with the above-mentioned device, in particular characterized in that tapping consists in a single momentary tapping pulse.
According to a further elaboration of the present method, it is used in a sorting device for eggs, wherein in particular the eggs are tapped at least twice. In a highly suitable manner, thus a suitable sorting criterion is created.